1. Field of the Invention
This invention relates to a flow resistance setting nozzle which may be set at a reference flow rate to be used for calibration of a flow measuring device, for example, and particularly contemplates providing a flow resistance setting nozzle which facilitates setting flow values and yet may maintain a particular set flow value for a long period of time to provide a highly reliable reference flow rate.
2. Description of the Related Art
The present inventors have heretofore proposed various flow resistance setting valves. One example of such valves illustrated in FIG. 6 shows a flow resistance setting valve proposed as xe2x80x9cmicro-leak valvexe2x80x9d in Japanese Utility Model Publication No. 5-42283 assigned to the same assignee of the present application. This flow resistance setting valve comprises a metallic block 1 having a tube receiving through-bore 3 for receiving a narrow tube 2 therethrough and a threaded hole 5-1 formed in the block so as to intersect the tube receiving through-bore 3 at a right angle, seal members 4A and 4B for forming seals between the outer periphery of the narrow tube 2 passed through the tube receiving through-bore 3 and the inner wall of the tube receiving through-bore 3, and a screw 5-2 threaded in the threaded hole 5-1, the arrangement being such that the narrow tube 2 can be squeezed by an appropriate amount by the screw 5-2 to set the cross-sectional area of the hollow bore of the narrow tube 2 at an appropriate value, whereby the flow of the gas which is allowed to pass through the narrow tube 2 may be restricted to obtain a desired reference flow rate.
With reference to FIG. 7, an example of this flow resistance setting valve in practical use will be explained. In FIG. 7 shown is a gas compression source 6 such as a compressor. The pressure of the gas compressed in the gas compression source 6 is regulated by a pressure regulator 7. The thus regulated gas pressure is governed by a pressure gauge 8-1 to be set at a constant gas pressure (primary pressure). The gas at the primary pressure is then provided to a flow resistance setting valve 9 as described with reference to FIG. 6. A secondary pressure of the gas output from the flow resistance setting valve 9 is measured at a second pressure gauge 8-2. When the pressure ratio of the secondary pressure to the primary pressure falls below a certain value, the velocity of flow of the gas passing through the restricted orifice of the flow resistance setting valve 9 will be equal to the sonic speed, and the gas is then provided at a constant restricted flow rate to a flow meter 10 to be calibrated. If the flow measurement value A of the flow meter 10 under calibration is calibrated so as to coincide with the flow rate set at the flow resistance setting valve 9, it means that calibration of the flow measurement value of the flow meter 10 under calibration is completed.
As is appreciated from the foregoing, the heretofore proposed flow resistance setting valve described above is configured to use the screw 5-2 for setting the flow rate. That is, the threaded amount of the screw 5-2 is adjusted to squeeze the narrow tube 2 so as to throttle the cross-sectional area of the narrow tube 2 whereby the flow rate of gas flowing at a predetermined given pressure may be set at a desired value.
With an increase in the threaded amount of the screw 5-2, the flow rate value is correspondingly turned down. However, if the flow rate value exceeds a target value by even a slight amount, the narrow tube 2 has to be replaced with a new one for readjusting the flow rate since the tube cannot be restored once it has been deformed. In this respect, it is to be appreciated that the prior art valve has the disadvantage that it is troublesome to set the flow rate.
In addition, the screw 5-2 takes the form of a screw commonly known as a kind of socket screw or socket set screw which is to be adjusted in position within the threaded hole 5-1. This is a type specifically designed in order to allow the threaded hole 5-1 to be sealed after completion of adjustment of the flow rate value, to thereby prevent the setting of the flow resistance from being easily tampered with once the flow rate has been set. It is for this reason that a set screw is used as the screw 5-2. However, the threaded shank of the set screw cannot be extremely reduced in diameter since the end face of the threaded shank is provided with a driver engagement groove engageable by a driver for turning the set screw for adjustment. In other words, the set screw usually has a relatively large diameter. Since as is known in the art the pitch of threads becomes more coarse with an increase in the diameter of the screw, it will be appreciated that it would be a difficult operation to set the flow rate at a target value by adjusting the screw 5-2 in the form of a set screw.
It is an object of this invention to provide a new flow resistance setting nozzle that is improved in easily and finely adjusting its flow rate to a desired target value.
In accordance with a first aspect of the present invention, a flow resistance setting nozzle is provided which comprises a metallic block having formed therethrough a pipe receiving through-bore; a metallic pipe for forming a flow path which is inserted in the pipe receiving through-bore; a pressing member in the form of a rectangular cross-section bar orthogonally abutting at a center position thereof with the pipe at a first abut portion thereof; a coarse adjustment screw mounted in the metallic block to press the pressing member at the center position thereof against the metallic pipe by a coarse pressing force generated by the coarse adjustment screw; an anvil having a distal end and mounted in the metallic block at a position opposite to the coarse adjustment screw such that the distal end of the anvil abuts the metallic pipe at a second abut portion thereof which is opposite to the first abut portion where the pipe abuts with the pressing member so that the pipe is received by and interposed between the pressing member and the anvil, whereby the pipe is squeezed by a coarse amount according to the coarse pressing force generated by the coarse adjustment screw; two fine adjustment screws mounted in the metallic block to press the pressing member at two symmetrically distant positions thereof which locate with substantially equal distances in longitudinally opposite directions of the pressing member from the center position thereof against the metallic pipe by a fine pressing force which is obtained from combination of respective pressing forces generated by the respective fine adjustment screws and which is additive to the coarse pressing force generated by the coarse adjustment screw, whereby the pipe is further squeezed by a fine amount according to the fine pressing force; and seal means for forming seals between the outer peripheral surface of the metallic pipe and the inner peripheral wall of the pipe receiving through-bore formed in the metallic block.
In accordance with a second aspect of the present invention, it is proposed in the flow resistance setting nozzle constructed according to the first aspect of the invention that the metallic block has a pressing member receiving bore which is formed therein to orthogonally intersect the pipe receiving through-bore, and that the pressing member is inserted in the pressing member receiving bore such that the pressing member orthogonally abuts with the metallic pipe at the first abut portion.
In accordance with a third aspect of the present invention, it is proposed in the flow resistance setting nozzle constructed according to the first or second aspect of the invention that each of the fine adjustment screws has a head portion and a threaded shank portion, that the pressing member has two through-apertures which are formed therethrough at the two symmetrically distant positions thereof for loosely receiving the threaded shank portions of the corresponding fine adjustment screws therethrough, and that the metallic block has two threaded holes which are coaxial with the two through-apertures and in which the threaded shank portions of the corresponding fine adjustment screws are threaded; whereby the fine pressing force is obtained from the combination of the respective pressing forces imparted by the head portions of the fine adjustment screws to the pressing member as the threaded shank portions of the fine adjustment screws loosely fitted in the through-apertures of the pressing member are threaded into the threaded holes in the metallic block.
In accordance with a fourth aspect of the present invention, it is proposed in the flow resistance setting nozzle constructed according to the third aspect of the invention that each of the threaded shank portion of the fine adjustment screws has a diameter smaller than that of a threaded shank portion of the coarse adjustment screw.
In accordance with a fifth aspect of the present invention, it is proposed in the flow resistance setting nozzle constructed according to either the first or second aspect of the invention that the distal end of the anvil has an arcuate surface with a top ridge thereof oriented in a direction orthogonal to the longitudinal direction of the pipe so that the arcuate surface of the anvil intersects the cylindrical peripheral surface of the pipe as the second abut portion.
In accordance with a sixth aspect of the present invention, it is proposed in the flow resistance setting nozzle constructed according to any of the first to fifth aspects of the invention that the metallic block has a coarse threaded hole and two screwdriving apertures formed therein, and that the respective head portions of the coarse adjustment screw and the two fine adjustment screws are recessed in the coarse threaded hole and the two screwdriving apertures formed in the metallic block, respectively.
In the flow resistance setting nozzle constructed according to the present invention, the provision of the fine adjustment screws in addition to the coarse adjustment screw facilitates the setting of the flow rate.
Further, in the flow resistance setting nozzle according to the present invention wherein the distal end of the anvil is provided with an arcuate surface oriented in a direction orthogonal to the cylindrical surface of the pipe, the restricted orifice portion of the pipe is progressively constricted axially of the pipe up to the most constricted section with a distance MS and then progressively expanded. Due to this orifice construction, the gas flowing through the pipe is allowed to pass the orifice smoothly to thereby attain a sonic flow rate value. In other words, the phenomenon in which the flow speed of the fluid reaches the sonic velocity at the restricted orifice to maintain a constant flow rate level if the pressure level on the primary side is set at a level above a predetermined value while the pressure level on the secondary side is set at a level below the predetermined value, is obtained.
It is thus to be understood that this invention allows for reproducing a highly reliable reference flow rate.